Silica removal from pregnant leach solutions

ABSTRACT

The present invention relates generally to a process for removing dissolved or colloidal silica from a pregnant leach solution (“PLS”). More particularly, an exemplary embodiment of the present invention relates to a, process which mixes PLS with an acid source, preferably lean electrolyte, to induce formation of colloidal silica that can then be collected and removed. Additionally, in an exemplary embodiment of the present invention, at least one silica seeding agent is added to induce formation of colloidal silica, at least one flocculant is added to induce aggregation of the colloidal silica, and a solid-liquid separation process is utilized to remove advantageous amounts or substantially all of the colloidal silica, thereby providing relief from supersaturation of dissolved silica in the metal recovery processes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims priority to U.S.application Ser. No. 12/718,796 filed Mar. 5, 2010, now U.S. Pat. No.8,114,365, and entitled “Silica Removal From Pregnant Leach Solutions.”The '796 application is a continuation of and claims priority to U.S.application Ser. No. 11/857,941 filed Sep. 19, 2007 and entitled “SilicaRemoval From Pregnant Leach Solutions,” now U.S. Pat. No. 7,691,347issued Apr. 6, 2010. All the aforementioned applications are herebyincorporated by reference in their entirety.

FIELD OF INVENTION

The present invention relates generally to a process for removingdissolved or colloidal silica from a pregnant, leach solution (“PLS”).More particularly, an exemplary embodiment of the present inventionrelates to a process which mixes. PLS with an acid source, preferablylean electrolyte, to induce formation of colloidal silica that can thenbe collected and removed. Additionally, in an exemplary embodiment ofthe present invention, at least one silica seeding agent is added toinduce formation of colloidal silica, at least one flocculant is addedto induce aggregation of the colloidal silica, and a solid-liquidseparation process is utilized to remove advantageous amounts orsubstantially all of the colloidal silica, thereby providing relief fromsupersaturation of dissolved silica in the metal recovery processes.

BACKGROUND OF THE INVENTION

Hydrometallurgical treatment of metal-bearing materials, such as metalores, metal-bearing concentrates, and other metal-bearing substances,has been well established for many years. Moreover, leaching ofmetal-bearing materials is a fundamental process utilized to extractmetals from metal-bearing materials. In general, the first step in thisprocess is contacting the metal-bearing material with an aqueoussolution containing a leaching agent which extracts the metal or metalsfrom the metal-bearing material into solution. For example, in copperleaching operations, especially copper from copper minerals, such aschalcopyrite and chalcocite, sulfuric acid in an aqueous solution iscontacted with copper-bearing ore. During the leaching process, acid inthe leach solution, may be consumed and various soluble components aredissolved thereby increasing the metal content of the aqueous solution.Other ions, such as iron may participate in the leaching of variousminerals as these ions participate in dissolution reactions.

Additionally, under these current leaching processes, especially copper,from copper sulfides such as chalcopyrite and chalcocite, largeconcentrations of dissolved silica are generated. This dissolved silicais gradually transformed to colloidal silica. Large amounts of thiscolloidal silica can agglomerate within process equipment, which maylead to inefficiencies in subsequent solvent extraction steps and lowoverall process yields. Additionally, this colloidal silica residue canresult in impurities in the extracted metal (i.e. impurities in metaldeposited during electrowinning steps).

Accordingly, a process that enables efficient metal recovery andprovides relief from supersaturation of dissolved silica in pregnantleach solutions, thereby reducing silica within the metal recoveryprocess, would be advantageous.

SUMMARY OF THE INVENTION

In general, according to exemplary embodiments of the present invention,the present invention relates generally to a process for removingdissolved or colloidal silica from a pregnant leach solution (“PLS”).More particularly, an exemplary embodiment of the present inventionrelates to a process which mixes PLS with an acid source, preferablylean electrolyte, to induce formation of colloidal silica that can thenbe collected and removed. Additionally, in an exemplary embodiment ofthe present invention, at least one silica seeding agent is added toinduce formation of colloidal silica, at least one flocculant is addedto induce aggregation of the formatted colloidal silica, and asolid-liquid separation process is utilized to remove advantageousamounts or substantially all of the colloidal silica, thereby providingrelief from supersaturation of dissolved silica in the metal recoveryprocesses.

For example, in accordance with the various embodiments of the presentinvention, the silica removal process can be implemented after anyreactive processing (discussed in greater detail hereinbelow), resultingin enhanced silica removal and various other advantages over prior artmetal recovery processes.

Additionally, in accordance with the various embodiments of the presentinvention, the reduction in the total dissolved silica and colloidalsilica in the PLS reduces impurities in the metal value deposited on thecathode during an electrowinning step and reduces colloidal silica inany subsequent solvent extraction step.

BRIEF DESCRIPTION OF THE DRAWING

A more complete understanding of the present invention, however, maybest be obtained by referring to the detailed description whenconsidered in connection with the figures, wherein like numerals denotelike elements and wherein:

FIG. 1 illustrates an exemplary flow diagram of a metal recovery processwith a silica removal circuit in accordance with one exemplaryembodiment of the present invention;

FIG. 2 illustrates an exemplary flow diagram of a silica, removalcircuit in accordance with one exemplary embodiment of the presentinvention;

FIG. 3 illustrates an exemplary flow diagram of a serial silica removalcircuit in accordance with one exemplary embodiment of the presentinvention;

FIG. 4 illustrates an exemplary flow diagram of a parallel silicaremoval circuit in accordance with one exemplary embodiment of thepresent invention;

FIG. 5 illustrates exemplary lab data where fumed silica is used as theseeding agent in accordance with an exemplary embodiment of the presentinvention;

FIG. 6 illustrates exemplary lab data where polyethylene oxide(PEO)-silica agglomerates are used as the seeding agent in accordancewith an exemplary embodiment of the present invention; and

FIG. 7 illustrates exemplary lab data for Galactosol-Ciba-silica is usedas the seeding agent in accordance with an exemplary embodiment of thepresent invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The detailed description of exemplary embodiments of the inventionherein shows various exemplary embodiments and the best modes, known tothe inventors at this time, of the invention are disclosed. Theseexemplary embodiments and modes are described in sufficient detail toenable those skilled in the art to practice the invention and are notintended to limit the scope, applicability, or configuration of theinvention in any way. Additionally, all included figures arenon-limiting illustrations of the exemplary embodiments and modes, whichsimilarly are not intended to limit the scope, applicability, orconfiguration of the invention in any way.

Various embodiments of the present invention exhibit significantadvancements over prior art processes, particularly with regard to metalrecovery and process efficiency. Moreover, existing metal recoveryprocesses that utilize a reactive process for metal recovery/solutionextraction/electrowinning process sequence may, in many instances, beeasily retrofitted to exploit the many commercial benefits the presentinvention provides.

Referring to FIG. 1, in accordance with various aspects of the presentinvention, a metal-bearing material 100 is provided for processing.Metal-bearing material 100 may be an ore, a concentrate, or any othermaterial from which copper and/or other metal values may be recovered.Metal values such as, for example, copper, gold, silver, zinc, platinumgroup metals, nickel, cobalt, molybdenum, rhenium, uranium, rare earthmetals, and the like, may be recovered from metal-bearing materials inaccordance with various embodiments of the present invention. Thevarious aspects and embodiments of the present invention, however, proveespecially advantageous in connection with the recovery of copper from,copper-bearing materials, such as, for example, ores and/or concentratescontaining chalcopyrite (CuFeS₂), chalcocite (Cu₂S), bornite (Cu₅FeS₄),and covellite (CuS), malachite (Cu₂CO₃(OH)₂), pseudomalachite(Cu₅[(OH)₂PO₄]₂), azurite (Cu₃(CO₃)₂(OH)₂), chrysocolla((Cu,Al)₂H₂Si₂O₅(OH)₄.nH₂0), cuprite (Cu₂O), brochanite(CuSO₄.3Cu(OH)₂), atacamite (Cu₂[OH₃Cl]) and other copper-bearingminerals or materials and mixtures thereof. Thus, metal-bearing material100 preferably is a copper ore or concentrate containing at least oneother metal value.

Metal-bearing material 100 may be prepared in conditioning step 201 formetal recovery processing in any manner that enables the conditions ofmetal-bearing material 100—such as, for example, composition andcomponent concentration—to be suitable for the chosen reactiveprocessing method, as such conditions may affect the overalleffectiveness and efficiency of metal recovery operations. Desiredcomposition and component concentration parameters can be achievedthrough a variety of chemical and/or physical processing stages, thechoice of which will depend upon the operating parameters of the chosenprocessing scheme, equipment cost and material specifications. Forexample, as discussed in some detail hereinbelow, metal-bearing material100 may undergo combination, flotation, blending, and/or slurryformation, as well as chemical and/or physical conditioning inconditioning step 201 before metal extraction.

In accordance with one aspect of the present invention, metal-bearingmaterial 100 may optionally be prepared in a conditioning step 201,wherein conditioning step 201 may comprise controlled, fine grinding.More precisely, U.S. Pat. No. 6,676,909 describing controlled grindingis contemplated herein and the subject matter of that patent is herebyincorporated by reference. Preferably, a uniform particle sizedistribution is achieved. It should be understood that a variety ofacceptable techniques and devices for reducing the particle size of thecopper-bearing material are currently available, such as ball mills,tower mills, grinding mills, attrition mills, stirred mills, horizontalmills, and the like, and additional techniques may later be developedthat may achieve the desired result of reducing the particle size of thecopper-bearing material to be transported.

Referring again to FIG. 1, in an exemplary embodiment of the presentinvention, after metal-bearing material 100 has been suitably preparedfor metal recovery processing, optionally by controlled grinding, andother physical and/or chemical conditioning processes 201, including butnot limited to a thickening process, it may be combined with any numberof liquid feed streams, including but not limited to process water, butany suitable liquid may be employed, such as, for example, recycledraffinate, pregnant leach solution (“PLS”), lean electrolyte, and/orother recycled streams from the metal recovery processes, including butnot limited to secondary metal, such as cobalt, iron, or manganese,recovery process streams, to form a metal-bearing inlet stream 101.

Moreover, in an exemplary embodiment of the present invention, aftermetal-bearing inlet stream 101 has been suitably prepared for metalrecovery processing, it may be forwarded to a reactive processing step202, for example, metal extraction. The reactive processing step 202 maybe any suitable process or reaction that puts a metal in themetal-bearing material 100 in a condition such that it may be subjectedto later metal recovery processing. For example, exemplary suitableprocesses include reactive processes that tend to liberate the desiredmetal value or values in the metal bearing material 100 from themetal-bearing material 100. In accordance with a preferred embodiment ofthe present invention, as described in greater detail below, reactiveprocessing step 202 may comprise a leaching process.

Furthermore, in an exemplary embodiment of the present invention, theleaching process may comprise any leaching process suitable forextracting the metal in metal-bearing material 100 into a PLS 102. Inaccordance with one aspect of the present invention, the leach stepcomprises atmospheric leaching, pressure leaching, agitation leaching,heap leaching, stockpile leaching, pad leaching, thin-layer leachingand/or vat leaching, at either ambient or elevated temperatures.Preferably, pressure leaching is a pressure leaching process operatingat a temperature in the range of about 140° C. to about 250° C. and morepreferably in the range of about 150° C. to about 220° C.

In accordance with an aspect of the present invention, the optimumtemperature range selected for operation will tend to maximize theextraction of copper and other metals, minimize acid consumption, andthereby minimize make-up acid requirements. That is, at highertemperatures, sulfide sulfur generally is converted to sulfate accordingto the following reaction:4CuFeS₂+17O₂+4H₂O→2Fe₂O₃+4Cu²⁺+8H⁺+8SO₄ ²⁻  (1)

At lower temperatures, acid is generally consumed and elemental sulfuris formed according to the following reaction:4CuFeS₂+8H⁺+5O₂→2Fe₂O₃+4Cu²⁺+8S°+4H₂O  (2)

Thus, in accordance with one aspect of the present invention, in orderto maintain preferable leaching temperature, a cooling liquid 301 may beintroduced into the leaching vessel during leaching. In accordance withone aspect of this embodiment of the present invention, a cooling liquid301 is preferably contacted with the feed stream in leaching vesselduring leaching. Cooling liquid 301 may comprise any suitable coolingfluid from within the process or from an outside source, such asrecycled liquid phase from the product slurry, make-up water, or amixture of cooling fluids. Cooling liquid may be introduced intoleaching vessel through the same inlet as metal-bearing inlet stream101, or in any manner that effectuates cooling of metal-bearing inletstream 101. The amount of cooling liquid added during leaching may varyaccording to the pulp density of the metal-bearing inlet stream 101, aswell as other parameters of the leaching process. In an exemplary aspectof this embodiment of the invention, a sufficient amount of coolingliquid 301 is added to reactive processing step 202 to yield a solidscontent in product slurry 102 on the order of less than about 50% solidsby weight, more preferably ranging from about 3 to about 35% solids byweight, and most preferably ranging from about 10% to about 20% solidsby weight.

Moreover, in accordance with one aspect of the present invention,reactive processing step 202 may occur in any pressure leaching vesselsuitably designed to contain the pressure leaching mixture at thedesired temperature and pressure conditions for the requisite pressureleaching residence time. In accordance with one aspect of an exemplaryembodiment of the invention, the pressure leaching vessel used inleaching step is an agitated, multi-compartment pressure leachingvessel. However, it should be appreciated that any pressure leachingvessel that suitably permits metal-bearing material 100 to be preparedfor metal recovery may be utilized within the scope of the presentinvention.

During reactive processing step 202, copper and/or other metal valuesmay be solubilized or otherwise liberated in preparation for laterrecovery processes. Any substance that assists in solubilizing the metalvalue, and thus releasing the metal value from a metal-bearing material,may be used. For example, where copper is the metal being recovered, anacid, such as sulfuric acid, may be contacted with the copper-bearingmaterial such that the copper may be solubilized for later recoverysteps. However, it should be appreciated that any suitable method ofsolubilizing metal values in preparation for later metal recovery stepsmay be utilized within the scope of this invention.

In accordance with one aspect of the present invention, reactiveprocessing step 202 comprises pressure leaching, sufficient oxygen 302is injected into a pressure leaching vessel to maintain an oxygenpartial pressure from about 75 to about 750 psi, preferably from about100 to about 400 psi, and most preferably from about 50 to about 200psi. Furthermore, due to the nature of medium temperature pressureleaching, the total operating pressure in leaching vessel 201 isgenerally superatmospheric.

The residence time for the pressure leaching process can vary, dependingon factors such as, for example, the characteristics of thecopper-bearing material and the operating pressure and temperature ofthe pressure leaching vessel. In one aspect of an exemplary embodimentof the invention, the residence time for the pressure leaching rangesfrom about 30 to about 180 minutes, more preferably from about 60 toabout 120 minutes.

Subsequent to metal-bearing material 100 undergoing reactive processingstep 202, the metal values that have been made available by reactiveprocessing step 202 undergo one or more of various conditioning steps203. In one exemplary embodiment, the product stream 102 from leachingstep 201 may be conditioned to adjust the composition, componentconcentrations, solids content, volume, temperature, pressure, and/orother physical and/or chemical parameters to desired values and thus toform a suitable metal-bearing solution. Generally, a properlyconditioned metal-bearing solution will contain a relatively highconcentration of soluble metal, for example, copper sulfate, in an acidsolution and preferably will contain few impurities. Moreover, theconditions of the metal-bearing solution preferably are keptsubstantially constant to enhance the quality and uniformity of thecopper product ultimately recovered.

In one aspect of an exemplary embodiment of the present invention,conditioning of a metal-bearing solution for metal recovery begins byadjusting certain physical parameters of the product slurry 102 from thereactive processing step 202. Optionally, in an exemplary aspect of thisembodiment of the invention, wherein the reactive processing step 202 ispressure leaching, it is desirable to reduce the temperature andpressure of the product slurry, in some instances to approximatelyambient conditions. An exemplary method of so adjusting the temperatureand pressure characteristics of the product slurry 102 is a conditioningstep 203 comprising flashing. In one aspect of an exemplary embodimentof the present invention, conditioning step 203 comprises atmosphericflashing. Further, flashed gases, solids, solutions, and steam mayoptionally be suitably treated, for example, by use of a Venturiscrubber wherein water may be recovered and hazardous materials may beprevented from entering the environment.

Under the current reactive and conditioning processes for metalrecovery, especially copper from copper sulfides such as chalcopyriteand chalcocite, large concentrations of dissolved silica are generated.This dissolved silica is gradually transformed into colloidal silica.Large amounts of this colloidal silica can agglomerate within processequipment, which may lead to inefficiencies in subsequent solventextraction steps and lower overall process yields. Additionally, thiscolloidal silica residue can result in impurities in the extracted metal(i.e. impurities in metal deposited during electrowinning steps 218).

Accordingly, the present invention teaches a process for relief fromsupersaturation of dissolved silica in PLS. Typical PLS can containbetween about 600 mg/L and about 1500 mg/L dissolved silica depending onthe reactive temperatures and/or processes utilized in the metalextraction processes. This dissolved silica can create impurities in thefinal metal product and systemic problems in the metal extractionprocess by creating colloidal silica, which can agglomerate withinprocessing equipment including, but not limited to tanks, pipes, andsolvent exchange apparatus. The silica removal process of the presentinvention can be implemented after any reactive processing, such as bymedium or high temperature pressure leaching, resulting in a PLS.

In accordance with an exemplary embodiment of the present invention, theprocess for providing relief from supersaturation of dissolved silica inPLS comprising: (i) providing a feed stream containing metal-bearingmaterial; (ii) subjecting at least a portion of the metal-bearing feedstream to at least one reactive process, wherein a PLS is formed; (iii)adding acid to the PLS; (iv) adding at least one seeding agent to thePLS; (v) forming colloidal silica from dissolved silica in the PLS; (vi)adding at least one flocculant to the formed colloidal silica, such thatthe colloidal silica agglomerates; (vi) removing at least a portion ofthe agglomerated colloidal silica; and (vii) recovering metal from theremaining PLS by electrowinning.

In accordance with another exemplary embodiment of the presentinvention, a process for recovering metal and providing relief fromsupersaturation of dissolved silica in PLS comprising: (i) providing afeed stream containing metal-bearing material; (ii) subjecting at leasta portion of the metal-bearing feed stream to at least one reactiveprocess, wherein a PLS is formed; (iii) adding acid to the PLS; (iv)adding at least one seeding agent to the PLS; (v) forming colloidalsilica from dissolved silica in the PLS; (vi) adding at least oneflocculant to the formed colloidal silica, such that the colloidalsilica agglomerates; (vii) removing at least a portion of theagglomerated colloidal silica; (viii) recovering metal from theremaining PLS by electrowinning; (ix) and providing at least a portionof the lean electrolyte from the electrowinning step to supply some orall of the acid used. In this way, the use of recycled acid-containingsolution, rather than concentrated sulfuric acid, is economicallyadvantageous.

In an exemplary embodiment illustrated by FIG. 1, after metal-bearingmaterial 100 has been suitably prepared for reactive processing, forexample, by other physical and/or chemical conditioning processes 201,optionally, controlled fine grinding, it is subjected to at least onereactive process step 202 to yield a PLS 102. By way of example,reactive process step 202 can be a high temperature or a mediumtemperature pressure leaching step. Preferably, pressure leaching is apressure leaching process operating at a temperature in the range ofabout 140° C. to about 250° C. and more preferably in the range of about150° C. to about 220° C. Most preferably, the pressure leaching processoperates at a temperature in the range of about 150° C. to about 160° C.

Further, referring again to FIG. 1, in one aspect of an exemplaryembodiment of the present invention, conditioning of a metal-bearingsolution after reactive process step 202 begins by adjusting certainphysical parameters in conditioning step 203. For example, as discussedin some detail herein below, after reactive processing 202 metal-bearingmaterial 100 may undergo reagent additions, flashing processes, one ormore solid-liquid phase separation steps including use of filtrationsystems, counter-current decantation (CCD) circuits, thickeners,clarifiers, or any other suitable device for solid-liquid separation, inconditioning step 203 to prepare the metal solubilized therein forrecovery.

In accordance with further aspects of this exemplary embodiment, theslurry product 102 from the reactive process step 202, or furtherconditioned slurry product stream 103, may be further conditioned inpreparation for later metal-value recovery steps in one or moresolid-liquid phase separation steps 204 may be used to separatesolubilized metal solution from solid particles. This may beaccomplished in any conventional manner, including use of filtrationsystems, counter-current decantation (CCD) circuits, thickeners,clarifiers, and the like. A variety of factors, such as the processmaterial balance, environmental regulations, residue composition,economic considerations, and the like, may affect the decision whetherto employ a CCD circuit, a thickener, a filter, a clarifier, or anyother suitable device in a solid-liquid separation apparatus. In oneaspect of an exemplary embodiment of the invention, one or moresolid-liquid phase separation steps 204 may be carried out with aconventional CCD utilizing conventional countercurrent washing of theresidue stream to recover leached metal values to one or more solutionproducts and to minimize the amount of soluble metal values advancingwith the solid residue to further metal recovery processes or storage.

Additionally, referring again to FIG. 1, in one aspect of an exemplaryembodiment of the present invention, the separated solids from one ormore solid-liquid phase separation steps 204 may further be subjected tolater processing steps, including secondary metal recovery, such as, forexample, recovery of gold, silver, platinum group metals, molybdenum,zinc, nickel, cobalt, uranium, rhenium, rare earth metals, and the like,by sulphidation, cyanidation, or other techniques. Alternatively, theseparated solids may be subject to impoundment or disposal.

Referring to FIG. 1, in an exemplary embodiment of the presentinvention, after PLS 104 has been suitably conditioned, in 203 or 204 itmay be forwarded to a desired metal recovery step. The metal recoverystep may include any suitable conditioning and/or copper recovery methodor methods, for example, electrowinning, formation, solution extraction(sometimes referred to as solvent extraction or liquid ion exchange),ion exchange, and/or ion flotation, and preferably results in arelatively pure copper product.

Further, referring again to FIG. 1, in an exemplary embodiment of thepresent invention, the resulting PLS 104 may be forwarded to a silicaremoval circuit 208. In accordance with an exemplary embodiment of thepresent invention as focused on the removal of silica, it is here, afterthe reactive processing steps 202, any optional conditioning steps 203,and/or any solid-liquid separation steps 204, when removal of dissolvedor colloidal silica is most advantageous.

Moreover, under normal medium-temperature pressure leaching conditions,the dissolved silica concentration of the PLS 102 is usually in therange of 300 mg/L to 400 mg/L, a concentration that exceeds thesolubility limit at room temperature. Over an extended period of timeand under these conditions, the dissolved silica monomer (Si(OH)₄)gradually polymerizes and transforms to colloidal particles, or silicagel. This colloidal silica can be responsible for silica agglomerationand silica residue throughout the metal extraction processes. The rateof polymerization is catalyzed by hydrogen ions and fluoride ions (ifany). When the dissolved silica experiences a higher acid concentration,the monomer polymerizes more quickly and precipitates as colloidalparticles according to:nSi(OH)₄

(SiO₂)_(n)+2H₂O

Generally, silica polymerization is often temperature dependant. Forexample, reducing the temperature increases the rate of silicapolymerization and vice versa.

In addition to dependence on temperature, the conversion from dissolvedsilica to colloidal silica is dependant on the acid concentration or pHof the PLS. Similarly, a decreased pH value will tend to give higherpolymerization rates. Thus, by controlling the pH, one has an additionaldegree of freedom for controlling the rate of silica polymerization. Forinstance, increasing the pH to about 2 will result in a sharp decreasein the rate of silica polymerization.

While dissolved silica is a problem in the metal extraction process, theuse of one or more solid-liquid phase separation steps areunsatisfactory to remove advantageous amounts of dissolved silica andcolloidal silica from the PLS 102. This is due in part because typicalmetal recovery processes do not form colloidal silica by addition of aseeding agent to assist in forming colloidal silica after anyconditioning steps, and prior to metal recovery, preferably byelectrowinning. Thus they do not remove advantageous amounts orsubstantially all dissolved silica, as the present invention does.

Accordingly, illustrated in FIG. 2 and in accordance with an exemplaryembodiment of the present invention, silica removal circuit 208 utilizesany process suitable for, causing silica dissolved in a pregnant leachsolution to form colloidal silica and removing the formed colloidalsilica. Preferably, in accordance with an exemplary embodiment of thepresent invention, silica removal circuit 208 utilizes the addition ofat least one seeding agent to assist in forming colloidal silica.

More specifically, again with reference to FIG. 2, in an exemplaryembodiment of the present invention, silica removal circuit 208 uses aprocess which PLS 104 with an acid source 108, preferably leanelectrolyte, to induce formation of colloidal silica that can then becollected and removed. Additionally, in an exemplary embodiment of thepresent invention, at least one silica seeding agent 109 is added toinduce formation of colloidal silica and a solid-liquid separationprocess is utilized to reduce colloidal silica throughout metal recoveryprocesses.

In an exemplary embodiment of the present invention with reference toFIG. 2, silica removal circuit 208 may comprise one or more mixing tanks210 suitable for mixing the PLS 104 with an acid source 108 to beginforming colloidal silica. Furthermore, in an exemplary embodiment of thepresent invention, silica removal circuit 208 may comprise one or moremixing tanks 210 suitable for mixing the PLS 104 with a seeding agent109 to begin forming colloidal silica. Additionally, in an exemplaryembodiment of the present invention, silica removal circuit 208 maycomprise one or more solid-liquid phase separation steps 214 to collectand/or remove colloidal silica. It should be understood, as will bediscussed in greater detail below, that causing dissolved silica to formcolloidal silica reduces the overall concentration of colloidal silicaresidue and agglomeration throughout metal recovery process, therebyenabling efficient metal recovery and providing relief fromsupersaturation of dissolved silica in the PLS.

More specifically, in an exemplary embodiment of the present inventionwith reference to FIG. 3 exemplifying silica removal circuit 208,colloidal silica can be formed from dissolved silica in PLS, 104, byfeeding PLS 104 to one or more mixing tanks 210, adding an acid source,107 or 108, wherein the acid source is preferably a fresh acid feed 107and/or a lean electrolyte recycle 108 from the electrowinning step 218.In accordance with an exemplary embodiment of the present invention, theacid, 107 or 108, is added in any amount suitable to induce theformation of colloidal silica, preferably greater than about 80 g/L ofacid is added. In accordance with an exemplary embodiment of the presentinvention, preferably 80 g/L to 180 g/L of acid is added.

In accordance with an exemplary embodiment of the present invention, theacid can be added to the PLS 104, either through the addition ofconcentrated H₂SO₄ and/or by blending PLS 104 with lean electrolyte (LE)108. In an exemplary embodiment, the acid is supplied from LE 108recycled from the electrolyte recycle 216 and/or electrowinning circuit218. Moreover, in accordance with an exemplary embodiment of the presentinvention, any PLS 104 to LE 107 volume ratio providing a acidconcentration greater than 80 g/L can be employed and are contemplatedin this disclosure. Based on a 225 g/L acid concentration of LE, inaccordance with an exemplary embodiment of the present invention, a 2:1PLS 104 to LE 107 volume ratio is preferable.

Also, in an exemplary embodiment of the present invention with referenceto FIG. 3, contemporaneous with or after the addition of acid, 107and/or 108, to PLS 104 at least one seeding agent, 109 and/or 110, maybe added, wherein the seeding agent is preferably a seeding agentsupplied by an external feed 109 and/or provided by a seeding agentrecycle from one or more solid-liquid phase separation steps 214.

Regarding the seeding agent, in accordance with an exemplary embodimentof the present invention illustrated in FIG. 3, at least one seedingagent, 109 and/or 110, is added into one or more mixing tanks 210 toincrease the rate of soluble silica transformation to the colloidalstate. Preferably, in one exemplary embodiment of the present invention,the concentration of the seeding agent added is 16 g/L or higher. Mostpreferably, in one exemplary embodiment of the present invention, theconcentration of the seeding agent added is 30 g/L or higher.

In accordance with an exemplary embodiment of the present inventionillustrated in FIG. 3, at least one seeding agent, 109 and/or 110, canbe provided from silica precipitates collected anywhere in the metalrecovery process (i.e. the electrowinning circuit 218—not shown); or byproviding external seeding agents 109 into the metal recovery process.For example, in an exemplary embodiment of the present invention,seeding agents, 109 and/or 110, can be any silica based seeding agentsincluding, but not limited to fumed silica, polyethylene oxide(PEO)-silica agglomerates, and/or a silica agglomerates formed bytreating colloidal silica rich PLS with Galactosol 40HD4CD (fromHercules, Inc.), guar gum, and Ciba 7689 (from Cytec Corporation).

For example, in an exemplary embodiment of the present invention, at 5%fumed silica and greater than 80 g/L acid, about 60-80% relief of silicasupersaturation was achieved within a 4 to 6 hour retention time. After24 hours, the extent of supersaturation relief was 80-90%, indicatingthat the bulk of the silica polymerization takes place within a 6 hourresidence time.

Similarly, in accordance with an exemplary embodiment of the presentinvention, the PEO-silica agglomerates were very effective in promotingsupersaturation relief, with faster polymerization at higher seedloadings. The concentration of PEO-silica agglomerates ranged from 4-40g/L (dry weight). The kinetics of the supersaturation relief process wasfast in the presence of excess acid (greater than 80 g/L acid). Between80 and 90% relief of supersaturation was attained within a 6 hourretention time. After a 24 hour retention time, the extent ofsupersaturation relief was 98-99%; thus, the bulk of the silicapolymerization takes place within a 6 hour residence time.

Additionally, in accordance with an exemplary embodiment of the presentinvention, silica agglomerates formed by treating colloidal silica richPLS with 30 mg/L Galactosol 40HD4CD and 10 mg/L Ciba 7689 resulted inSimilar kinetics when added as seed material. In a 6 hour residencetime, about 80% of the supersaturated silica was relieved from solution;in a 24 hour residence time, 95% of the excess soluble silica wasremoved from solution by the seed material, again indicating that thebulk of the polymerization chemistry takes place within a 6 hourresidence time at an operating temperature of 50° C. Additionally, therate of supersaturation relief was found to be independent oftemperature within the 25-80° C. range.

In accordance with an exemplary embodiment of the present inventionillustrated in FIG. 3, the seeding agent can be repeatedly recycled fromdifferent parts of the metal extraction process, wherever colloidalsilica is formed. For example, in one embodiment of the presentinvention, the seeding agent can be recycled from the solid-liquidseparation process, 214 (with reference to FIG. 2, FIG. 3, and FIG. 4)and/or the electrowinning effluent 108 (with reference to FIG. 1).

In an exemplary embodiment of the invention with reference to FIG. 2 andFIG. 3, the PLS 104, acid, 107 and/or 108, and at least one seedingagent, 109 and/or 110, are fed into one or more mixing tanks 210. Thismixture is then mixed and/or stored for a predetermined amount ofresidence time to induce further nucleation and formation of colloidalsilica. In accordance with one embodiment, this mixture can betransferred between multiple mixing tanks 210 in series or parallel foradditional mixing and residence time to induce further nucleation andformation of colloidal silica. Furthermore, in accordance with anotherexemplary embodiment of the present invention, after the acid, 107and/or 108, and at least one seeding agent, 109 and/or 110, are mixedwith the PLS 104 and colloidal silica is formed, the colloidal silicaslurry 112 may be forwarded to one or more reagent dosing tanks 212 anda flocculant 113 may be added. In accordance with another exemplaryembodiment of the present invention, flocculant 113 may any substancethat promotes flocculation by causing silica, colloids and/or othersilica particles in the colloidal silica slurry 112 to aggregate, orform floccules.

For example, in accordance with another exemplary embodiment of thepresent invention, flocculant 113 may comprise any multivalent cation,including but not limited to any aluminum, iron, calcium, and/ormagnesium, or any polymer, including but not limited to polyacrylamides,compound suitable for promoting flocculation of colloidal silica.Furthermore, in accordance with another exemplary embodiment of thepresent invention, flocculant 113 may comprise at least one ofpolyethylene oxide (PEO), Galactosol 40HD4CD, guar gum, Ciba 7689, andScifloc C2733.

After sufficient colloidal silica has been formed and agglomerated bythe flocculant 113 in reagent dosing tank 212, in accordance withanother exemplary embodiment of the present invention, the flocculatedslurry 114 may be forwarded to one or more solid/liquid separation steps214 to remove advantageous amounts of colloidal silica. As mentioned,one or more solid-liquid phase separation steps 214 may be used toseparate flocculated colloidal silica and to form metal-rich solution111 prepared for metal recovery processes. This solid-liquid phaseseparation may be accomplished in any conventional manner, including useof filtration systems, counter-current decantation (CCD) circuits,thickeners, clarifiers, centrifuges, and the like. In accordance withfurther aspects of this preferred embodiment, solid-liquid phaseseparation step 214 comprises a dissolved-air flotation (DAF),pinned-bed clarification, column flotation, air-encapsulatedflocculation, vibrating Sweco screening, stationary Kason screening, orTrommel screening.

In another exemplary embodiment of the present invention, theflocculated slurry 114 is fed to the solid/liquid separation step 214 ata temperature less than about 100° C. Most preferably, in anotherexemplary embodiment of the present invention, the flocculated slurry114 is fed to the solid/liquid separation step 214 at a temperaturegreater than 25° C. and less than about 85° C., most preferably 50° C.

Preferably, in another exemplary embodiment of the present invention,between all the mixing tanks, 210, the solution is mixed continuously orintermittently for four (4) hours or more and allowed to be aged for aresidence time of six (6) hours to remove an advantageous amount ofsilica from the metal extraction process. It should be understood thatnumerous variations on mixing times and aging or residence times arecontemplated within this invention.

In one exemplary embodiment of the present invention, removingadvantageous amounts of colloidal silica means removing more silica thanwould be removed in a solid/liquid separation step without the additionof a seeding agent at the same temperature, acidity, and with, the samemixing and residence time.

In one exemplary embodiment of the present invention, removingadvantageous amounts of colloidal silica means removing greater thanabout 60% of the total silica in the metal extraction process. Inanother exemplary embodiment of the present invention, removingadvantageous amounts of colloidal silica means removing greater thanabout 70% of the total silica in the metal extraction process. Inanother exemplary embodiment of the present invention, removingadvantageous amounts of colloidal silica means removing greater thanabout 90% of the total silica in the metal extraction process. Inanother exemplary embodiment of the present invention, removingadvantageous amounts of colloidal silica means removing about 98% of thetotal silica in the metal extraction process.

In an alternative exemplary embodiment with reference to FIG. 4, afterthe PLS 104 is mixed with acid, 107 and/or 108, and a seeding agent, 109and/or 110, in one or more mixing tanks 210, as described above.Preferably, in accordance with another exemplary embodiment of thepresent invention, PLS 104 is mixed with acid, 107 and/or 108, and aseeding agent, 109 and/or 110, in two or more mixing tanks, 220 and 222,in parallel with one another. Similarly, in accordance with anotherexemplary embodiment of the present invention, after the acid, 107and/or 108, and at least one seeding agent, 109 and/or 110, are mixedwith the PLS 104 and the desired amount of colloidal silica is formed,the colloidal silica slurry 115 from mixing tank 220 and colloidalsilica slurry 116 from mixing tank 222 may be forwarded to one or morereagent dosing tanks 212 and a flocculant 113 may be added. The twoexemplary configurations described herein and depicted in the drawingsare serial and parallel configurations, respectively, and numerousconfigurations are contemplated within the scope of this disclosure toremove dissolved silica and colloidal silica from the PLS.

With regard to FIG. 3 and FIG. 4, in an exemplary embodiment of thepresent invention, the flocculated slurry 114 now contains concentratedamounts of flocculated colloidal silica and substantially less dissolvedsilica is transferred to the solid/liquid separation step 214 to removethe colloidal silica and, thus, metal-rich solution, or rich electrolytesolution 111, is forwarded to the electrolyte recycle tank 216.Preferably, in an exemplary embodiment, the final solid-liquidseparation step 214 is a thickener with most of the underflow 110recycled to enhance formation and removal of colloidal silica.Experimental results for some of the exemplary processes are provided inthe Example Section below.

In addition to the underflow or bottoms of the solid/liquid separationstep 214 being recycled to one or more mixing tanks, 210, 220, and/or222, the bottoms can be split into a residue stream, including removedcolloidal silica, 117 (FIG. 3) or 127 (FIG. 4), and depending on thestream, 117 (FIG. 3) or 127 (FIG. 4), composition, may be neutralized,impounded, disposed of, or subjected to further processing, such as, forexample, precious metal recovery, treatment to recover other metalvalues, such as, for example, recovery of gold, silver, platinum groupmetals, nickel, cobalt, molybdenum, zinc, rhenium, uranium, rare earthmetals, and the like. Optionally, in accordance with an exemplaryembodiment illustrated in FIG. 1 and FIG. 2, a portion of the residuestream, including removed colloidal silica, 117 (FIG. 3) or 127 (FIG.4), can be recycled to other steps of the metal recovery process.

Lastly, again with reference to FIG. 1, in an exemplary embodiment,after one or more silica removal circuit 208, metal-rich solution, orrich electrolyte solution 111 is substantially free from supersaturatedsilica. The stream may then be sent to electrolyte recycle tank 216.Electrolyte recycle tank 216 suitably facilitates process control forelectrowinning circuit 218, as will be discussed in greater detailbelow. Metal-containing solution stream 111, is preferably blended witha lean electrolyte stream 108 in electrolyte recycle tank 216 at a ratiosuitable to yield a product stream 118, the conditions of which may bechosen to optimize the resultant product of electrowinning circuit 218.With continued reference to FIG. 1, metal from the product stream 118 issuitably electrowon to yield a pure, cathode metal product 120.

For the sake of convenience and a broad understanding of the presentinvention, an electrowinning circuit useful in connection with variousembodiments of the invention may comprise an electrowinning circuit,constructed and configured to operate in a conventional manner. Theelectrowinning circuit may include, electrowinning cells constructed aselongated rectangular tanks containing suspended parallel flat cathodesof metal alternating with flat anodes of lead alloy, arrangedperpendicular to the long axis of the tank. A metal-bearing leachsolution may be provided to the tank, for example at one end, to flowperpendicular (referring to the overall flow pattern) to the plane ofthe parallel anodes and cathodes, and metal can be deposited at thecathode and water electrolyzed to form oxygen and protons at the anodewith the application of current. Other electrolyte distribution and flowprofiles may be used.

The primary electrochemical reactions for electrowinning of metal fromacid solution is believed to be as follows:2CuSO₄+2H₂O→2Cu°+2H₂SO₄+O₂Cathode half-reaction: Cu²⁺+2e ⁻→Cu°Anode half-reaction: 2H₂O→4H⁺+O₂+4e ⁻

Turning again to FIG. 1, in a preferred embodiment of the invention,product stream 118 is directed from electrolyte recycle tank 216 to anelectrowinning circuit 218, which contains one or more conventionalelectrowinning cells. It should be understood, however, that any methodand/or apparatus currently known or hereinafter devised suitable for theelectrowinning of metal from acid solution, in accordance with theabove-referenced reactions or otherwise, is within the scope of thepresent invention.

In accordance with a preferred aspect of the invention, electrowinningcircuit 218 yields a cathode metal product 120, optionally, an off gasstream (not shown), and a relatively large volume of metal-containingacid solution, herein designated as lean electrolyte stream 121. Asdiscussed above, in the illustrated embodiment of the invention, aportion of lean electrolyte stream 121 (FIG. 1) is preferably recycledto various places in the metal extraction process including, but notlimited to reactive processing step 202 and/or to electrolyte recycletank 216. Optionally, a portion of metal-containing solution stream 111from silica removal circuit 208 is combined with lean electrolyterecycle stream 108 and is recycled to reactive processing step 202.Moreover, in accordance with one aspect of an exemplary embodiment ofthe invention, a portion of lean electrolyte stream 121 (leanelectrolyte bleed stream 119 in FIG. 1) is removed from the metalrecovery process, exemplified in FIG. 1, for the removal of impuritiesand, acid and/or residual metal recovery operations.

Preferably, lean electrolyte recycle stream 108 comprises at least about50 percent by weight of lean electrolyte stream 121, more preferablyfrom about 60 to about 95 percent by weight of lean electrolyte stream121, and most preferably from about 80 to about 90 percent by weight oflean electrolyte stream 121. Preferably, lean electrolyte bleed stream119 comprises less than about 50 percent by weight of lean electrolytestream 121, more preferably from about 5 to about 40 percent by weightof lean electrolyte stream 121, and most preferably from about 10 toabout 20 percent by weight of lean electrolyte stream 121.

Metal values, from the metal-bearing product stream 120 are removedduring electrowinning step 218 to yield a pure, cathode metal product.It should be appreciated that in accordance with the various aspects ofthe invention, a process wherein, upon proper conditioning of themetal-bearing solution, a high quality, uniformly-plated cathode metalproduct may be realized without subjecting the metal-bearing solution tosolvent/solution extraction prior to entering the electrowinning circuitis within the scope of the present invention. As previously noted,careful control of the conditions of the metal-bearing solution enteringan electrowinning circuit—especially maintenance of a substantiallyconstant metal composition in the stream—can enhance the quality of theelectrowon metal by, among other things, enabling even plating of metalon the cathode and avoidance of surface porosity in the cathode metal,which degrades the metal product and thus may diminish its economicvalue. In accordance with this aspect of the invention, such processcontrol can be accomplished using any of a variety of techniques andequipment configurations, so long as the chosen system and/or methodmaintain a sufficiently constant feed stream to the electrowinningcircuit. A variety of methods and apparatus are available for theelectrowinning of metal and other metal values, any of which may besuitable for use in accordance with the present invention, provided therequisite process parameters for the chosen method or apparatus aresatisfied.

The Example set forth hereinbelow is illustrative of various aspects ofa preferred embodiment of the present invention. The process conditionsand parameters reflected therein are intended to exemplify variousaspects of the invention, and are not intended to limit the scope of theclaimed invention.

ACID ADDITION EXAMPLES Example 1

Aged (3-week-old) medium-temperature PLS containing 330 mg/L total Siwas obtained. After 6 weeks of aging and maintaining the PLS at 50° C.,the soluble silicon concentration decreased to 143 mg/L and was blendedwith LE. After blending with 223 g/L of acid at a 1:2.5 volume ratio andtreating with 20 mg/L PEO, the soluble silicon concentration was reducedfrom 75 to 34 mg/L. The addition of the LE catalyzed the relief ofsilica supersaturation. This example indicates that concentration ofacid is crucial to remove advantageous amounts of colloidal silica.

Example 2

Concentrated H₂SO₄ was added to fresh PLS samples and allowed to ageover a three day period. Samples were taken periodically and treatedwith the equivalent of 20 mg/L PEO to remove colloidal silica. After 10to 15 min at 50° C., treated samples were filtered on 0.45 μm media.Filtrates were analyzed for soluble silica (as silicon). Table 1 showsthat increasing the acid concentration of the blend solution promotesremoval advantageous amounts of colloidal silica.

TABLE 1 Soluble Si Acid Analyses, mg/L Experiment ID Addition, g/L Day 1Day 3 2938-133-1 0 165 158 2938-136-1 117 154 101 2938-136-2 143 152 822938-136-3 182 149 75 2938-136-4 219 145 76 2938-136-5 257 142 71

USE OF SEEDS TO FORM COLLOIDAL SILICA EXAMPLES Example 1

Samples of fresh PLS were slurried with fumed silica to promote therelief of silica supersaturation. The slurries were stirred at 50° C.over a period of time. Samples were taken at 1, 2, 4, 6, and 24 hours,filtered, and analyzed for soluble silica (as silicon). FIG. 5 shows theextent of silica supersaturation relief from PLS, with and without LEaddition, when fumed silica is used as seed. The rate of the processincreases with increasing solid load and/or acid concentration.Comparison with data generated show that the supersaturation reliefkinetics was fast with fumed silica, most likely due to the higherspecific surface area. At 5% solids loading, 60-80% relief ofsupersaturation was achieved in a 4- to 6-hr retention time. After a24-hr residence time, close to 80% relief of supersaturation wasachieved in the presence of LE with sufficient acid. The data shows thatfumed silica can be successfully used as seed material to removeadvantageous amounts of colloidal silica.

Example 2

Fresh silica agglomerates generated from 10-15 mg/L PEO treatment ofcolloidal silica-rich LE was recycled as seed material to relievesupersaturation of silica from fresh PLS with and without LE addition.The slurries were stirred at 50° C. over a period of time. Samples weretaken at 1, 2, 4, 6, and 24 hours, filtered, and analyzed for solublesilica (as silicon). There was no further PEO treatment of thesolutions.

FIG. 6 illustrates that 80-90% relief of supersaturation was attainedwithin a 6-hr retention time. After 24-hr contact, 98-99% relief ofsupersaturation was attained. The data shows that agglomerated silicafrom PEO treatment of LE can be recycled as seed, material to fresh PLSto remove advantageous amounts of colloidal silica.

FIG. 7 shows the extent of supersaturation relief achieved when therecycled seed material was generated by treating colloidal silica-richLE with a combination of 30 mg/L Galactosol•40HD4CD and 10 mg/L Ciba7689. The data show that the silica agglomerates generated from thetreatment of LE with combined Galactosol 40HD4CD and Ciba 7689 initiatedsoluble silica removal effects similar to those of the PEO-silicaagglomerates; greater than 80% and 95% supersaturation relief wasattained after 6 and 24 hours, respectively. The data generated fromthese experiments suggest that Galactosol and Ciba-coagulated silicaagglomerates, can be recycled as seed material to remove advantageousamounts of colloidal silica from a blend of fresh PLS and LE withoutprolonged aging of the PLS.

SOLID-LIQUID PHASE SEPARATION EXAMPLES Example 1

A method for encapsulating air during flocculation was attempted in 16batch scoping experiments, feed solution assaying 158 mg/L total Si, wasused for these experiments. High-speed mixing created a large vortexcapable of temporarily suspending fine air bubbles in solution. Whenflocculant was added to the stirred solution, the suspended air bubbleswere trapped, within the silica floccules. When agitation was stopped,the resultant floccule masses instantly rose to the top of the solutionand remained stable for many hours after formation.

An initial experiment used a 1 g/L stock solution of Polyox WSR-301(PEO)at doses of 10, 20, and 30 mg/L of solution, and gave post-flocculationsolution silica values of 96, 44, and 30 mg/L Si, respectively.Subsequent testing with freshly prepared 1 g/L WSR-301 solution at 10and 20 mg/L dosages gave solution silica values of 86 and 66 mg/L Si,respectively.

Other PEO formulations used were Ucarfloc 302, 304, and 309 (“UCF”),which correspond to increasing molecular weights. Each of theformulations was tested at doses of 10 and 20 mg/L. These showedreductions in silica concentrations with increasing doses. UCF-309 gavethe best results, resulting in a treated solution concentration of 38mg/L Si after a polymer dose of 20 mg/L. This example illustrates thatair-encapsulated flocculation is possible method to remove advantageousamounts of colloidal silica.

Example 2

A pilot-size stationary Kason screen with an adjustable screen angle wasemployed to process PEO treated LE to recover PEO-silica agglomerates.The screening was continuous and was fed with two mix tanks in series toprovide an approximately 3-min retention time in each tank. A 10-mg/LPEO dose, which was effective in bench-top experiments, gave a silicaconcentration of 75 mg/L Si. This example illustrates that Kason screenis a possible method to remove advantageous amounts of colloidal silica.

Example 3

A continuous bench-top trommel screen was constructed to test recoveryof PEO-silica agglomerates from LE.

The initial experiments showed that the liquor passing through thescreen was very clear, indicating little remaining colloidal silica.This example illustrates that trommel screen is a possible method toremove advantageous amounts of colloidal silica.

Example 4

After 22 recycles, the slurry was transferred to a 1-L graduatedcylinder equipped with a mechanical rake, spiked with the equivalent of4 mg/L PEO, and subjected to a settling procedure.

The settling characteristics of the slurry are summarized in Table 2.This example illustrates that thickening is a possible method to removeadvantageous amounts of colloidal silica.

TABLE 2 Feed PEO Settled Solids Initial Thickener Slurry Solids Solids,Overflow TSS, Settling Capacity, Solids, % Dose, g/t % mg/L Rate, m/hrm2/(t/day) 1.8 190 13.5 37 0.7 4.98

As used herein, the terms “comprise”, “comprises”, “comprising”,“having”, “including”, “includes”, or any variation thereof, areintended to reference a non-exclusive inclusion, such that a process,method, article, composition or apparatus that comprises a list ofelements does not include only those elements recited, but can alsoinclude other elements not expressly listed and equivalents inherentlyknown or obvious to those of reasonable skill in the art. Othercombinations and/or modifications of structures, arrangements,applications, proportions, elements, materials, or components used inthe practice of the instant invention, in addition to those notspecifically recited, can be varied or otherwise particularly adapted tospecific environments, manufacturing specifications, design parametersor other operating requirements without departing from the scope of theinstant invention and are intended to be included in this disclosure.

Moreover, unless specifically noted, it is the Applicant's intent thatthe words and phrases in the specification and the claims be given thecommonly accepted generic meaning or an ordinary and accustomed meaningused by those of reasonable skill in the applicable arts. In theinstance where these meanings differ, the words and phrases in thespecification and the claims should be given the broadest possible,generic meaning. If it is intended to limit or narrow these meanings,specific, descriptive adjectives will be used. Absent the use of thesespecific adjectives, the words and phrases in the specification and theclaims should be given the broadest possible meaning. If any otherspecial meaning is intended for any word or phrase, the specificationwill clearly state and define the special meaning.

The use of the words “function”, “means” or “step” in the specificationor claims is not intended to invoke the provisions of 35 USC 112,Paragraph 6, to define the invention. To the contrary, if suchprovisions are intended to be invoked to define the invention, then theclaims will specifically state the phrases “means for” or “step for” anda function, without recitation of such phrases of any material,structure, or at in support of the function. Contrastingly, theintention is NOT to invoke such provision when then claims cite a “meansfor” or a “step for” performing a function with recitation of anystructure, material, or act in support of the function. If suchprovision is invoked to define the invention it is intended that theinvention not be limited only to the specific structure, materials, oracts that are described in the preferred embodiments, but in addition toinclude any and all structures, materials, or acts that perform theclaimed function, along with any and all known or later-developedequivalent materials, structures, or acts for performing the claimedfunction.

What is claimed is:
 1. A method for providing relief from dissolvedsilica in pregnant leach solutions comprising: providing a meal-bearingsolution comprising a metal value and dissolved silica; introducing anacid and a seeding agent to a the metal-bearing solution to yield ametal bearing solution comprising the metal value and colloidal silica;adding a flocculant to the metal-hearing solution such that thecolloidal silica agglomerates; removing at least a portion of theagglomerated silica.
 2. The method of claim 1, wherein the seeding agentis a colloidal silica seeding agent.
 3. The method of claim 1, whereinthe seeding agent comprises a portion of the removed agglomeratedsilica.
 4. The method of claim 1, wherein the step of removing at leasta portion of the agglomerated silica comprises providing themetal-bearing solution comprising agglomerated silica to a solid-liquidseparation apparatus, wherein the solid-liquid separation apparatuscomprises at least one of a filtration system, a countercurrentdecantation circuit, a thickener, a centrifuge, a screen, a flotationmethod, a clarification method, and a flocculation method.
 5. The methodof claim 1, further comprising a step of providing the metal-bearingsolution to an electrowinning process after a portion of theagglomerated silica is removed.
 6. The method of claim 5, wherein thecolloidal silica seeding agent is recycled from the electrowinningprocess.